U.S. Pat. Nos. 4,311,826, 4,348,313 and 4,391,969 disclose sulfur cement compositions comprising elemental sulfur and a plasticizer. The plasticizer is usually present in an amount of approx. 5% and comprises a mixture of dicyclopentadiene and oligomers of cyclopentadiene in a ratio of 1:1. U.S. Pat. No. 4,293,463 discloses a sulfur cement composition comprising sulfur, a viscosity-increasing, surface-active, finely divided particulate stabilizer, such as fly ash, and an olefinic hydrocarbon polymeric material derived from petroleum and having a non-volatile content larger than about 50% by weight and having a minimum Wijs iodine number of about 100 cg/g, said polymer being capable of reacting with sulfur to form a sulfur-containing polymer, (cf. column 2, lines 29-37). Corresponding sulfur cement compositions are described in U.S. Pat. No. 4,058,500.
Sulfur cement is used to replace conventional cement for the preparation of concrete. In contrast to conventional portland cement concrete the obtained sulfur concrete is distinguished by being resistant to salt and acid attacks. Sulfur cement is the only binder when preparing sulfur concrete. Sulfur cement becomes liquid when heated to a temperature of up to approx. 135.degree. C. thus rendering the sulfur concrete as workable as conventional concrete. During the cooling process usually lasting only a few hours the sulfur concrete develops unique properties, such as compressive strengths of approx. 40 to 60 MPa, flexural strengths of approx. 8 to 12 MPa, high impact strength, high wear resistance, absolute water impermeability, resistance to salt and acid attacks as well as resistance to freeze thaw exposure. Examples of sulfur concrete and its preparation are disclosed in U.S. Pat. Nos. 4,025,352, 4,496,659, 4,332,911 and 4,332,912.
Due to these excellent properties sulfur concrete has been applied in many fields of industry dealing with aggressive environment, where conventional portland cement concrete disintegrates. Typical fields of application, where the particular properties of sulfur concrete are of special interest, include the manufacture of floors, coatings, foundations, walls, acid reservoirs, tanks, sewer systems and the like.
Although there are many good reasons for using sulfur concrete no acceptable method for the manufacture of sulfur concrete pipes from sulfur concrete has yet been found. Various methods for the manufacture of such pipes have indeed been suggested, but none of them can be considered satisfactory from a technical and economical point of view. Examples of known methods for the manufacture of sulfur concrete articles are mentioned in the following patent specifications.
U.S. Pat. No. 3,954,480 discloses concrete compositions, concrete articles and methods of producing the articles, where a portion of the cement is replaced by sulfur, preferably plasticized sulfur. The patent thus discloses a combination of sulfur concrete and conventional concrete. During the production of the concrete articles the material is shaped into the desired shape and is compacted, whereupon the product is left to stand in order to permit partial or complete hydration of the cement therein. The hydratized product is then heated to a temperature above the melting point of sulfur to plastify the sulfur, whereupon the article is cooled and ready to be used. This method is very time-consuming since the partial or complete hydratization of the cement requires a long time and the heat treatment is usually carried out during heating for between 1 and 5 h at a temperature of between 121.degree. and 177.degree. C. Moreover, due to the content of conventional cement, it is impossible to obtain all the above advantages of sulfur cement.
U.S. Pat. No. 4,134,775 discloses articles to be used as structural members, such as bricks, building blocks, mouldings, cornices or the like substantially comprising a 3-dimensional matrix of solidified elemental sulfur and a solid, particulate, inorganic material uniformly distributed throughout the matrix, said particulate, inorganic material amounting to 20 to 80% by weight of the article. At least one part of the particulate, inorganic material is unfragmented fly ash amounting to 20 to 60% by weight of the article. The particulate, inorganic material has a particle size in the range of from 0.0005 to 10.0 mm, the maximum particle size being small in comparison to the smallest dimension of the finished article. The article is manufactured by mixing sulfur concrete at room temperature and pouring the mixture into a mould. The mixture is then heated until the sulfur has melted. The necessary strength of the article is obtained during the subsequent cooling. Such an article has a hardness significantly greater than the one of solidified sulfur and a compressive strength higher than the one obtained with aged, cast concrete. The produced articles can also be extruded in form of pellets to be remelted for in situ use. The method disclosed in U.S. Pat. No. 4,134,775 is time-consuming and only applicable in connection with small articles and can thus not be used for the manufacture of pipes on an industrial scale.
U.S. Pat. No. 4,256,499 discloses shaped sulfur concrete articles and their production from a mouldable composition of mineral aggregate, mineral binder, a sulfur component, such as elemental sulfur, and a liquid vehicle, such as water. The mixture is compacted and formed into a shaped body at an elevated compacting pressure, dried to volatilize the liquid vehicle and heated to melt the sulfur. The article is subsequently cooled to solidify the sulfur, whereby the mineral materials are bonded into a matrix with the sulfur. The use of liquid vehicle renders the method inconvenient and time-consuming, as subsequent to compacting and casting the material has to be dried until substantially all liquid vehicle is volatilized.
U.S. Pat. No. 4,426,458 discloses fiber-reinforced sulfur concrete compositions, which according to the specification are proposed for the preparation of concrete articles, such as paving slabs, structural members, curbings, gutters and pipes. The sulfur concrete composition comprises sulfur cement, aggregate and fiber elements in form of bundles of filaments with a length of at least 3 cm. The aggregate may have the following distribution of particle sizes:
15-80% by weight of a particle size larger than 4.75 mm, preferably 1.5 to 4 mm, in diameter PA0 5-85% by weight of a particle size of between 150 .mu.m and 4.75 mm in diameter PA0 5-15% by weight of a particle size of less than approx 150 .mu.m in diameter.
The preparation of the sulfur concrete articles according to this specification may be carried out by mixing the preheated aggregate with melted sulfur and fiber elements in a mixer at between approx. 120 and 140 C. Subsequently the hot mixture is cast. The patent specification deals exclusively with the problems connected with fiber-reinforcing. The specification does not contain any reference allowing the industrial-scale manufacture of pipes by a person skilled in the art.
EP Patent application No. 0,048,106 Al discloses sulfur compositions including sulfur concrete, comprising particulate, inorganic aggregate bonded together in a matrix of the sulfur component having a plurality of small, entrained cells. According to the specification it is suggested that articles, such as paving slabs, structural members, curbings, gutters, pipes and the like, are to be manufactured by casting such sulfur concretes. It is, however, not described how to manufacture pipes. The entrained cells can be admixed the sulfur component in various ways during its preparation, said cells comprising a gas, such as air, oxygen, nitrogen, carbon dioxide or halocarbons, or a finely divided, porous, particulate material. In a preferred embodiment of the preparation of sulfur concrete the inorganic aggregate is first preheated to a temperature ranging from approx. 115.degree. to approx. 160.degree. C, whereupon it is mixed with the liquid sulfur cement in a suitable mixer until a substantially homogeneous mixture is obtained, the temperature being maintained throughout the mixing. The hot mixture is subsequently cast using a conventional plant. The moment for introducing the small entrained cells depends on the cell-entraining method employed. This patent application discloses methods for the admixing of air but does not teach how to manufacture sulfur concrete pipes on an industrial scale.
The disadvantage of the preparation of the above and other sulfur concrete articles is in general that the molten sulfur concrete is of low viscosity. This may cause problems connected with for example the necessity for a large number of moulds, or cleaning the moulds between two casts due to the material sticking to the mould walls, or shrinkage resulting in incorrect final dimensions, or precipitation of aggregate or segregation of aggregate and molten sulfur during the cooling period. It has therefore been the general opinion of people skilled in the art that the manufacture of sulfur concrete pipes requires a special plant and particular handling.
The above opinion among people skilled in the art that the manufacture of pipes from sulfur concrete is a very difficult task, has been further supported by the general knowledge among those skilled in the art that 10 years ago a Canadian manufacturer went bankrupt in a futile attempt at manufacturing sulfur concrete pipes. A U.S. manufacturer was also forced to abandon the manufacture of sulfur concrete pipes after an investment of 1 mio U.S.-dollars.
Moreover, at the International Sulfur Concrete Symposium & Workshop, arranged by The Sulfur Institute, Oct. 14-15, 1986, Washington, D.C., Alfred Ecker (from OMV Aktiengesellschaft, Austria) communicated that "aggregate grading according to the specifications of portland cement concrete is unsatisfactory for sulfur concrete". With respect to the possibility of manufacturing pipes from sulfur concrete he communicated that the manufacture by means of the centrifugal method caused problems with segregation and internal tensions. Further A. Ecker communicated that "prefabrication of sulfur concrete parts seems to be very easy, but casting, vibrating and mould construction require extensive experience in that sulfur concrete is a thermoplastic material for which specialized handling is necessary". "The transmission of sulfur concrete technology from laboratory to commercial production is difficult and expensive". At this symposium the lecturers Thomas A. Sullivan (Consultant) and William McBee (U.S. Bureau of Mines) expressed corresponding opinions.
Several methods for the manufacture of pipes from portland cement concrete are known, for example the centrifugal method requiring complex equipment, the so-called wet method requiring long setting periods prior to demoulding and the dry casting method. The dry casting method involves vigorous vibration thus tightly compacting the solid particles in such a way that portland cement concrete pipes can be demoulded immediately after casting.
Hardening of portland cement concrete pipes is due to a chemical reaction between portland cement and water as well as optional pozzolanas and additives. Usually 28 days are considered necessary for complete hardening. Thus the portland cement concrete pipes have a low strength during the first few days subsequent to casting.
Portland cement concrete pipes are very sensitive to drying out, especially during the first days after casting since the chemical reaction requires water. As a result the quality of the finished portland cement concrete pipes varies greatly depending on the hardening conditions.
Portland cement concrete pipes are not 100% waterimpermeable, since portland cement concrete has always a more or less closed capillary pore system. Aggressive substances, such as salts and acids, easily penetrate the concrete and disintegrate it.
Portland cement concrete pipes do thus not resist acid attacks, for example by sulfuric acid. Due to various bacteria hydrogen sulfide is frequently formed in large sewer system. On the top part of a sewer pipe hydrogen sulfide is oxidized to sulfuric acid, said acid rapidly corroding and disintegrating portland cement concrete pipes. This problem occurs particularly in warm climates. It is, for example, necessary to exchange the entire sewer system of Los Angeles.
Portland cement concrete for the manufacture of pipes can be prepared from a mixture of fine aggregate (aggregate particles of less than 4 mm) and cement or a mixture of fine aggregate and coarse aggregate (aggregate particles of more than 4 mm) and cement. Typical compositions comprise:
______________________________________ portland cement 200-400 kg/m.sup.3 fly ash 0-150 kg/m.sup.3 fine aggregate 800-1800 kg/m.sup.3 coarse aggregate 0-1000 kg/m.sup.3 water 120-140 kg/m.sup.3 ______________________________________
Moreover various chemical and mineral additives can be employed.
Contrary to sulfur concrete, portland cement concrete cannot be cast at temperatures below 0.degree. C. due to the presence of water.
Today methods are known to coat portland cement concrete pipes with, for example, epoxy. Such methods are, however, expensive and connected with various disadvantages. Owing to the health risk involved in handling epoxy, such as its carcinogenous properties, safety requirements are extensive. Moreover it is not easy to ensure a sufficient impermeability of the coating. This also applies to another form of coating, where the concrete pipe is lined with plastic material. In this case great problems concerning impermeability arise especially where the pipes are joint.
In Denmark as well as in all other industrial countries there is an increasing demand for corrosion-resistant pipes in connection with the exchange of existing pipe systems as well as for new systems.
As mentioned above it has not yet been possible to manufacture sulfur concrete pipes in a satisfactory manner in such a way that such pipes can be used in areas where conventional portland cement concrete pipes are not resistant enough. There is thus a great demand for sulfur concrete pipes manufactured in an economic and effective manner, such as by using existing conventional plants for the manufacture of portland cement concrete pipes.